Paint coatings are applied in the manufacture and finishing of a vast range of goods and products including metal sheet and coil, timber flooring, automobiles, aeroplanes, fencing, glass, packaging and the like. In the area of metal sheet and coil, the substrate is typically galvanized steel, aluminium, stainless steel, or zinc alloy coated steels including aluminium-zinc and zinc-magnesium coated steels.
Devices are available to measure the paint coating thickness on these products to avoid costly wastage of the paint coating material, and to ensure the quality of the finished product. These devices include magnetic sensors, ultrasonic sensors, and mechanical-optical devices such as the commercially available DJH gauge system (DJH Designs Inc. Oakville, ON, Canada). This system involves mechanically boring a shallow crater through a painted surface into the underlying substrate of a sample. The sample is then placed under a microscope and the crater is viewed on a high resolution monitor for determination of the paint coating thickness. Likewise, paint coating thickness can be determined by sectioning a sample cut from a coated product and viewing the sample under high magnification. Other methods include the use of a micrometer to measure thickness of the coated article, stripping off the paint coating, re-measuring the article, and calculating the paint coating thickness as the difference between the two readings. However, the above techniques either have limited accuracy, or they are very time consuming and labour intensive. Moreover, they do not measure or monitor the thickness of the wet paint coating during the paint coating process for determination of the final dry paint coating thickness.
Lasers have also been used in the past for the measurement of paint coating thickness, for example, as described US 848,785 (EP 2 031 347 A1). The method described in EP 2031347 requires knowledge of the original thickness/dimensions of the substrate before the paint coating is applied to the substrate, in order to calculate the thickness of the paint coating using a laser sensor. These measurements are static measurements, and the thickness/dimensions of the substrate are known in advance. In particular, a laser sensor is used to measure the size of the substrate before it is coated, and to monitor the paint coating thickness during the application/deposition of the paint coating to determine the end point for the paint coating deposition process. However, the methods and apparatus described EP 2031347 are not suitable for the measurement of opaque paint coatings of greater than about 4 μM thickness, when the paint coatings are applied to a substrate of unknown size, or are moving at speeds of greater than about 15 meters per minute during an industrial paint coating process.